This invention relates to a method and apparatus for control of the gas pressure in a superplastic forming apparatus, and more particularly to a gas control system and method of operation for precise control of the forming gas pressure and a method for accurately measuring the strain rate of a metal blank in a superplastic forming apparatus.
Superplastic forming is a metal forming process used within the aerospace industry and elsewhere for manufacturing single sheet detailed parts and multi-sheet built-up structures. The design flexibility that is offered by superplastic forming has resulted in substantial cost savings in the fabrication of the detailed parts and assemblies. In the aircraft industry, additional benefits are in superior quality control, reduce weight savings, and lower part variability.
The superplastic forming process utilizes a phenomenon of certain alloys of metal which, when heated to a certain temperature, are capable of undergoing enormous plastic elongation (or strain) with uniform thinning throughout the full area of the metal blank. The processes normally practiced in industry includes heating a die to the superplastic temperature of the metal alloy and placing the metal blank in the die. A restraining pressure is exerted on the die lid to hold it closed against the forming pressure of the gas which is injected into the die above the metal blank to form it into the die cavity in the die base. After the forming is completed, the gas pressure is relieved, the die is opened and the finished part is removed from the die. This basic process is disclosed in U.S. Pat. No. 3,934,441 to Howard Hamilton, et al.
The basic superplastic forming process has been improved for particular metals and to achieve enhanced capabilities in the last twenty or so years. A process known as back pressure forming has been developed for preventing cavitation in certain aluminum alloys. Cavitation is a phenomenon characteristic of certain alloys, 7475 aluminum in particular, in which micro-voids coalesce and propagate from the middle of the sheet, severely degrading the strength properties of the formed part. By applying back pressure during the forming of a part, the cavitation can be considerably minimized or eliminated altogether. The process of applying back pressure is initiated at the beginning of the forming cycle by ramping the pressure on both sides of the sheet to desired pressure and then increasing the pressure above the sheet to form it into the die cavity. This process is disclosed in U.S. Pat. No. 4,354,369 entitled "Method for Superplastic Forming" issued to Howard Hamilton.
Superplastic forming/diffusion bonding is a process in which a pack of two or more sheets are bonded together by means of a diffusion bond at the point of contact and gas is injected between the sheets of the pack to inflate the pack in a die to take the shape of the cavity of the die. This process is ideal for creating a sandwich panel having two face sheets and internal diagonal supporting structure to couple the two sheets in a strong and lightweight integral structure. Superplastic forming/diffusion bonding is disclosed in U.S. Pat. No. 3,927,817 entitled "Method for Making Metallic Sandwich Structures" issued to Howard Hamilton.
The basic superplastic forming process and the several variants of the basic process all require precise control of the gas pressure used to apply forming of pressure to strain the superplastic material in the right direction.
Although superplastic forming has proven to be a valuable and successful manufacturing process, it has not met the original expectations that existed for it when the process was first being explored. One of the primary reasons for the problems that have been experienced in the use of superplastic forming is in the nature of the superplastic process itself. Each superplastic material has an ideal temperature and strain rate at which it can be formed. Deviations from these ideal conditions produce less than optimum results, and sometimes unsatisfactory results altogether. One of the primary reasons for the unsatisfactory results is inability to strain the material at the optimum strain rate. The total elongation, hence the depth of draw of the material, is very much dependent on the strain rate at which the material is formed. If the material is formed too quickly (at to great a strain rate) it may rupture or tear before the forming is finished. It may also undergo work hardening and lose its plasticity. A slow strain rate lengthens the forming cycle and decreases the throughput through the machine. It can also result in grain growth resulting in a decrease of the total possible elongation that can be achieved with that material. A relationship has been devised to quantify this phenomenon and is typically termed "strain rate sensitivity" or "M value" for a particular material. The strain rate at which the material is strained is almost entirely a function of the forming gas pressure and therefore the control over that gas pressure is critical to the optimal utilization of the SPF process.
Since the strain rate at which the material is formed is such a critical parameter to optimum use of the SPF process, it would be desirable to provide a technique for measuring what the actual strain rate is at any particular moment during the forming process. In the past, the strain rate has been calculated from the known M value of the material and the pressure of the forming gas and the configuration of the part. However, the strain rate is not a well enough understood function of the numerous complex factors that influence the strain rate, and so the calculated strain rate is only an approximation of the strain rate actually achieved in practice. It would be of great value to those working in the field to have a process for accurately measuring what the actual strain rate is at any given moment to enable them to precisely tailor the pressure cycle of the forming gas used to form the part in the press.
For superplastic forming back pressure and superplastic forming/diffusion bonding, the forming gas pressure is likewise critical to the process. The pressure on the side of the sheet away from which it is forming must always be at a higher pressure than the side of the sheet toward which it is forming, and the differential pressure must be carefully controlled so that the forming rate is at the optimum forming rate. This requires that the pressure on one side of the sheet be known and that the differential pressure likewise be known so that the back pressure and the forming pressure can both be controlled accurately to produce the optimum results.